Catalytic asymmetric hydrocarboxylation and hydrohydroxymethylation. A two-step approach to the enantioselective functionalization of vinylarenes

A method for the catalytic asymmetric hydrocarboxylation and hydrohydroxymethylation of vinylarenes is reported. By separating the step in which asymmetry is installed from the one where carbon-carbon bond formation takes place, a highly enantioselective and regioselective synthesis of Ibuprofen and its analogs is achieved. Stereochemistry is installed via an enantioselective hydroboration reaction, catalyzed by cationic rhodium BINAP complexes, and the homologation is carried out with halomethyllithium reagents. If CH2Cl2/n-BuLi is used as the homologating reagent, carboxylic acids are produced after oxidation. On the other hand, if CH2ClBr is used in combination with n-BuLi, the corresponding primary alcohol is produced. This latter transformation represents a previously unknown asymmetric hydrohydroxymethylation reaction. In both cases, complete retention of stereochemistry is observed, yielding products in up to 97% ee. Superior results are obtained if the initially formed catecholate is converted into a pinacolate prior to homologation.